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Design and Simulation of a Micro Piezoelectric
Energy Harvester Based on a Mass Proof Cantilever
F. A. C. Oliveira1, D. W. de Lima Monteiro1
1
Laboratory for Optronics and Microtechnology Applications (OptMAlab) – Department of Electrical Engineering Universidade Federal de Minas Gerais (UFMG)
e-mail: [email protected]
1. Abstract
A micro piezoelectric vibrational energy harvester
based on a mass proof cantilever with a thin aluminium
nitride film was designed and simulated. The fabrication
strategy, design parameters and simulation results were
presented and an evaluation of its applicability is
discussed.
2. Introduction and Motivation
The continuous improvement of electronic devices
has enabled the development of sophisticated ultra-low
power systems with integrated sensors, low range
wireless transmitters and controlling processing units.
The research for alternative methods for powering these
systems led to the development of micro vibrational
energy harvesters. The use of piezoelectric materials has
a significant advantage for this application, due to the
direct conversion of mechanical stress in electrical
charge, and the use of aluminium nitride (AlN) enables
an easy integration with microfabrication processes.
The proposed energy harvester design was made for
the MEMSCAP PiezoMUMPs process [1], offered in a
multi wafer project manner to enable low-cost
prototyping of piezoelectric micro-electro-mechanicalsystems (MEMS) devices. The process is based on a
silicon on insulator (SOI) wafer and consists of five
lithographic steps. Structures in the top silicon layer can
be released with a through-wafer etch of the bottom
silicon layer. An AlN thin film is deposited on top of the
top silicon, and a conductive metal film can be
deposited over it, acting as a top electrode and electrical
path. The top silicon is used for the patterning of the
mechanical structures and also acts as bottom electrode.
A thermal oxide film is used for interlayer isolation.
generating charge that flows as an electric current to the
circuit to be powered. In this way, the energy generated
depends, among many other factors, on the size of the
tip mass, the geometrical parameters of the cantilever,
the frequency and intensity of the environmental
vibrations and the input load of the powered circuit.
In order to effectively harvest the environment
vibrational energy the device must be designed with a
low resonant frequency, matching the higher energy
harmonics frequencies in typical sources of mechanical
vibrations [8]. For that reason, the aimed resonant
frequency for the designed structure was below 200Hz.
The weight of the tip mass, the increased length and
the reduced width of the cantilever contributes to a
lower resonant frequency [6]. The PiezoMUMPs
process imposes a maximum released mass of 1x1mm
and due to sub-die dimensional constraints, the length of
the cantilever was limited to 3200µm. A conservative
width of 400µm was chosen in order to avoid premature
mechanical failure of the silicon beam. The
PiezoMUMPs process sets the thicknesses of the layers
as follows: 400µm bottom silicon; 1µm buried oxide;
10µm top silicon; 0.5µm AlN; 1.015µm metal layer
(0.015µm of chrome with 1µm aluminium on top).
A tri-dimensional model of the designed structure is
shown in Fig.1. The calculated fundamental resonant
frequency of 147.41Hz was obtained disregarding the
effects of the thin films on top of silicon. This value was
validated by a finite-elements method (FEM) simulation
using COMSOL Multiphysics software.
3. Device Operation and Design
The mass proof cantilever has been proved as an
effective design for vibrational energy harvesting by
numerous authors [2, 3, 4, 5, 6] and the AlN has been
compared with PZT (a material with a much higher
piezoelectric coefficient) showing theoretical better
results for this application [7].
The oscillation of the mass in the free tip of the
cantilever, due to the ambient mechanical vibrations,
promotes a high stress in the anchored end of the
structure where the piezoelectric film is deposited,
Fig.1. Device structure with dimensions in µm.
4. Simulation
The proposed design performance was simulated
using a simple one dimensional model based on a
piezoelectric accelerometer [9]. The specific AlN
parameters were extracted from measurements
published in relevant articles using similar AlN
deposition processes. The actual parameters are highly
dependent on the process recipe and can only be
accurately determined once the fabricated device is
tested, but for estimation purposes the parameters values
used will suffice. The Table I below lists those.
Table I. Material parameters values and references.
Parameters
Monocrystalline silicon density
and elasticity coefficient
AlN transversal piezoelectric
coefficient, resistivity and
relative dielectric coefficient
Dry air viscosity and density
Value
2320kg/m³
160GPa
-2.8pC/N
252MΩm
10
17.66µNs/m²
1.134kg/m³
Reference
[10]
[11]
[12]
[13]
[14]
5. Discussion and Conclusion
The ambient vibration acceleration used is of
moderate intensity and can be found in industrial
environments. The total generated power achieved by
fitting the maximum of 6 devices in a PiezoMUMPs
regular-size die, considering optimal conditions, would
be of 2.18µW. This is enough to feed a low-power
wireless sensor system. Auxiliary circuitry such as
rectifiers and impedance matching circuits should be
used in a real application and that shall be discussed in
future work.
Acknowledgments
A special thanks to CNPQ and FAPEMIG, who
respectively financed the research scholarship and the
fabrication of the device chip, currently ongoing.
References
The generated power has a complex relation with
many parameters, but it is directly proportional to the
square of the acceleration of the ambient vibrations; to
the tip mass; to the match between the device resonant
frequency and the ambient vibrations frequency; and to
the inverse of the damping coefficient. The damping
coefficient was modeled using Navier-Stokes equations,
having multiple parcels related to the different sources
of damping in the structure [15].
Considering a moderate intensity source of
environmental vibration, with an acceleration of 4m/s²
in the resonant frequency of the device, the generated
power and output voltage relates to the resistive
electrical load of the powered circuit as shown in Fig. 2.
Fig.2. Plot of generated power and voltage vs load resistance
for ambient vibrations of 4m/s² of intensity in 147.41Hz.
A maximum of 363nW power generation occurs for
the optimal load of 7.61MΩ, with a voltage of 1.66V.
The relationship of the generated power and the optimal
load resistance with the ambient vibration frequency
was also simulated and it will be presented in another
work.
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